1. Field of the Invention
The present invention relates to multi-level modulation receiving devices, and more particularly, to a multi-level modulation receiving device for receiving a multi-level modulated optical signal.
2. Description of the Related Art
As a result of the recent growing tendency toward longer-distance, higher-capacity optical transmission, dispersion compensation technology for compensating for dispersion caused in optical fibers has become an indispensable technique. Optical transmission systems with transmission rates of the order of 10 Gbps generally employ a chromatic dispersion compensation method in which a dispersion compensating fiber (DCF) is incorporated in an optical amplifier-repeater which is arranged on the transmission line at regular intervals.
In the case of 10-Gbps optical transmission systems, the dispersion compensation method using DCFs can, in many cases, satisfactorily compensate for chromatic dispersion that varies with temperature and with time, to an extent such that the BER (Bit Error Rate) falls within an allowable range.
However, where higher-speed modulation is carried out to attain higher-speed optical transmission exceeding 10 Gbps, for example, 40-Gbps or faster optical transmission, chromatic dispersion exerts a more serious influence on signals than in the case of 10-Gbps optical transmission systems. Besides, the influence of polarization mode dispersion, which poses rather small problem in the case of 10-Gbps transmission, becomes significant.
Specifically, the influence of chromatic dispersion grows stronger in proportion to the square of the transmission rate, and therefore, the allowable range of chromatic dispersion at 40 Gbps is as narrow as 1/16 (e.g., about 40 ps/nm or less) of the allowable range at 10 Gbps. On the other hand, the influence of polarization mode dispersion heightens in proportion to the transmission rate, and accordingly, the allowable range of polarization mode dispersion at 40 Gbps is as narrow as ¼ of the allowable range at 10 Gbps.
Thus, 40-Gbps high-speed optical transmission systems require higher-precision dispersion compensation techniques, and the key issue is how to suppress the waveform degradation caused by chromatic dispersion and polarization mode dispersion.
Chromatic dispersion is a phenomenon wherein different wavelength components in an optical signal have different propagation times. Generally, chromatic dispersion is expressed as a propagation time difference obtained when two monochromatic light beams with a wavelength difference of 1 nm are propagated over 1 km.
Polarization mode dispersion signifies a phenomenon wherein two orthogonal polarizations have different propagation constants because of slight birefringence caused in a transmission line constituted by an optical fiber or the like. In the case of an optical fiber, for example, polarization mode dispersion does not occur if the fiber core has a perfectly circular cross-section. However, in real optical fibers, birefringence is caused due to slight ellipticity or asymmetric stress. Polarization mode dispersion includes the first-order polarization mode dispersion and higher-order polarization mode dispersions.
A transmission line with birefringence exhibits slightly different refractive indices depending on the incident polarization, and an optical fiber transmission line equivalently possesses a slow axis (optical path with a large refractive index through which light travels at low speed) and a fast axis (optical path with a small refractive index through which light travels at high speed). A propagation time difference between the slow and fast axes is referred to as the first-order polarization mode dispersion. The amount of waveform distortion of an optical signal caused by the first-order polarization mode dispersion depends also on in what ratio light enters the slow and fast axes.
In the case of an optical fiber transmission line extending over a long distance, the two, slow and fast axes are not regarded as extending straight throughout the transmission line. Instead, such a long transmission line is modeled in a manner such that an interval with its own slow and fast axes and another interval with its own slow and fast axes are interconnected with a certain gradient. Thus, a long transmission line is a concatenation of a large number of intervals with their own slow and fast axes.
In such transmission lines, complex waveform degradation occurs as channels of different optical wavelengths undergo respective different slow and fast axes or even a single wavelength channel undergoes different slow and fast axes depending on the optical frequency component because the modulation spectrum contains high- and low-frequency components. These phenomena are called higher-order polarization mode dispersions.
Where the transmission distance is long, the first-order polarization mode dispersion sometimes accumulates up to about several picoseconds to ten-odd picoseconds. In 10-Gbps transmission, one timeslot is about 100 ps long, and therefore, in the case of 10-Gbps or slower optical transmission systems, a polarization mode dispersion of several picoseconds poses no significant problem since it falls within the allowable range. In 40-Gbps optical transmission, however, the length of one timeslot is ¼ of that of a timeslot for 10-Gbps transmission. Consequently, a polarization mode dispersion of several picoseconds to ten-odd picoseconds becomes a main cause of inter-symbol interference.
As conventional optical transmission techniques, there have been proposed multi-level modulation optical transmission techniques in which multi-level modulated optical signals are transmitted and received so as to suppress waveform degradation attributable to dispersion (e.g., PCT-based Unexamined Japanese Patent Publication No. 2004-516743 (paragraph nos. [0018] to [0037], FIG. 1); non-patent documents: “Electrical signal processing techniques in long-haul fiber-optic systems,” IEEE Transactions on Communications, Vol. 38, No. 9, pp. 1439-1453, September 1990, and “An APD/FET optical receiver operating at 8 Gbit/s,” Journal of Lightwave Technology, Vol. LT-5, No. 3, March 1987).
Meanwhile, optical networks are under study and development which employ a redundant configuration for optical fiber transmission lines and which have an optical protection switching function whereby, if an optical fiber transmission line develops a fault, switching to a standby path is executed.
In cases where optical signals are transmitted at 40 Gbps across such an optical network, the tolerance to chromatic dispersion at 40 Gbps is very narrow, as stated above. Accordingly, if there is even a slight difference of chromatic dispersion between current and standby optical fiber transmission lines, the dispersion difference becomes outside the dispersion tolerance range at the time of switching the optical fibers. Also, in the case of transmitting, for example, SDH (Synchronous Digital Hierarchy) signals over an optical network, it is necessary to meet the requirement that the time required until the SDH optical path switching should not exceed 50 ms.
In connection with the polarization mode dispersion, if the optical fiber transmission line is subjected to vibration (e.g., an aerial cable is vibrated by wind or a cable laid along a railroad track is vibrated due to the passage of a railroad car) or to physical external force or displacement (e.g., a person working in an office touches the transmission line), the polarization state of the optical signal propagated through the transmission line changes (light propagated through the slow axis until then is propagated through the fast axis or the optical branching ratio of the slow and fast axes varies), with the result that the waveform distortion varies with time.
Accordingly, to construct 40-Gbps redundant optical networks with the protection switching function, there has been a strong demand for adaptive, high-precision dispersion compensation techniques capable of remedying the waveform degradation caused by chromatic dispersion and polarization mode dispersion, while following the time-varying waveform degradation in a short time of several milliseconds or less.